Distance measuring devices have become known in which an ultrasonic signal is at first transmitted via an electroacoustic converter for a particular time. Thereafter, when the said ultrasonic signal has been reflected on an obstacle, it is received from the same converter as an echo signal. From the time difference between transmission of the ultrasonic signal and reception of the echo signal the distance of the obstacle from the converter can be computed. Devices of this kind are used in a motor vehicle, in order to facilitate maneuverizing in narrow parking space to minimize the risk of colliding with another vehicle. Such devices for measuring the distance between motor vehicles therefore ought to have a detection range from about 1.5 m to 0.25 m. Undesired false signals such as those caused by ground echos must be minimized. Furthermore in devices of this kind, target obstacles must be reliably detected over the entire above mentioned range of detection independently of their reflection properties.
An important problem in developing such distance measuring devices is that after activation of the converter a significant time passes in which the converter output decays. Although the amplitude of the signal measured at the converter decreases relatively rapidly, the measured signal considerably exceeds the component contributed by echo signals. Thus echo signals which may hit the converter during the decay time, cannot be recognized. Thus in a system of this kind the lower limit of the measuring range, in which the converter serves to transmit ultrasonic signals and simultaneously to receive echo signals, is dependent on the decay time of the converter. However the decay time is dependent on various factors. In addition to construction tolerances, influences of temperature play an important role.
Consequently known constructions in which, during a defined time from the beginning or from the end of the activation of the converter, the receiver is blocked, do not show fully satisfactory operational properties. If the time, in which the receiver is blocked, is indeed relatively long and thus an evaluation of the echo signals is prevented, the lower limit of the measuring range will not reach values in the order of magnitude of 0.25 m. If, however, this time is reduced, it can happen with unfavorable influences, that an echo signal is pretended and evaluated by the converter still decaying.